(1) Field of the Invention
The present invention relates to starch-polyester graft copolymers. The present invention particularly relates to a chemically modified starch-polyester graft copolymer and a chemically modified starch-nano clay polyester graft copolymer. A process of making these graft copolymers using reactive processing such as reactive extrusion is disclosed. The starch-polyester graft copolymers have improved processability, surface properties, and an extended range of mechanical properties.
(2) Description of Related Art
Plastic packaging is subjected to pressure from existing and proposed environmental and disposal regulations, and market based sustainability initiatives. It presents a major disposal problem for companies and municipalities as it is lightweight and bulky and so does not lend itself to a viable economic and environmentally responsible recycling operation due to expensive handling and transportation costs. It is not biodegradable, which makes disposal in soil or composting operations untenable. Further, issues such as sustainability, industrial ecology, biodegradability, and recyclability are becoming major considerations in a company's product packaging design, especially with single use disposable packaging. Natural biopolymers provide biodegradable, sustainable solutions for the manufacture of short-life, single use disposable packaging, consumer goods, and marine plastics. Starch, an anhydroglucose polymer, offers a structural platform to manufacture sustainable, biodegradable packaging. Examples of patents that disclose the manufacture of starch and blends of starch and other polymers include U.S. Pat. No. 4,673,438 to Wittwer et al.; U.S. Pat. No. 4,095,054 to Lay et al.; U.S. Pat. No. 5,256,711 to Tokiwa et al.; U.S. Pat. No. 5,275,774 to Bahr et al.; U.S. Pat. No. 5,382,611 to Stepto et al. and U.S. Pat. No. 5,405,564 to Stepto et al. Lately, there have been business reports of the use of thermoplastic starch (TPS) as a component in multi phase blends (W. Wiedmann, and E. Strobel, Starch, 43, 138 (1991); R. L. Shorgen, G. F. Fanta, and W. M. Doan, Starch, 45, 276 (1993); P. Forssell, J. Mikkila, and T. Sourtti, J.M.S. Pure Appl. Chem., A33, 703 (1996); R. Narayan, Polymers from Agricultural Co products, ACS Symp Ser. (1994); and J. J. G. Van Soest, K. Benes, and D. de. Wit, Polymer, 37, 3543 (1996). Still others have manufactured thermoplastic starch blends in which native starch is initially blended with a small quantity of water and a less volatile plasticizer such as glycerin in order to form starch melts that are subjected to a degassing procedure prior to cooling and solidification in order to remove substantially all of the water therefrom. Examples of such patents include U.S. Pat. Nos. 5,412,005, 5,280,055, 5,288,765, 5,262,458, 5,462,980 and 5,512,378 to Bastioli et al.
Starch granules exhibit hydrophilic properties and strong inter-molecular association via hydrogen bonding due to the hydroxyl groups on the granule surface. The hydrophilicity and thermal sensitivity render the starch polymer unsuitable for thermoplastic applications. In this respect, some authors have emphasized on finding the optimal polymer or mixture of polymers and other admixtures in order to thereby “optimize” the properties of the starch. One drawback is that most of the polymers and other admixtures are themselves significantly more expensive than starch, which tends to increase the cost of such polymer blends compared to starch melts. Another drawback is that such additives will only be able to marginally alter the mechanical properties of the starch/polymer blends when viewed from a materials science perspective.
In order to improve on these drawbacks, graft copolymerization of vinyl monomer on the starch backbone was used to modify starch. Fanta and Bagley have reviewed the synthesis and discussed some applications of starch graft copolymers (G. F. Fanta and E. B. Bagley, Encyclopedia of Polymer Science, John Wiley & Sons: New York (1970); and G. F. Fanta, Block and Graft Copolymers—Vol I, John Wiley & Sons: New York (1973). Otey et al. (F. H. Otey, R. P. Westhoff and W. M. Doane, Industrial Engineering Chemistry Products Research Development, 19, 592 (1980); F. H. Otey and R. P. Westhoff, Industrial Engineering Chemistry Products Research Development, 23, 284 (1984); and F. H. Otey, R. P. Westhoff and W. M. Doane, Industrial Engineering Chemistry Products Research Development, 26, 1659 (1987)) blended starch with poly(ethylene-co-acrylic acid) (EAA). In these papers, the authors suggested the formation of hydrogen bonds between the carboxylic group in EAA and the hydroxyl group in starch. Increasing the level of starch decreased the percentage elongation of the film and increased the diffusion rate of water. Similar complexes like EAA can also be formed with the hydroxyl groups of the polyethylene-vinyl alcohol (EVOH) copolymer. They report a reaction between the anhydride group in the synthetic polymer with the —OH groups of starch. U.S. Pat. No. 5,462,983 to Bloembergen at al. reports on blends and alloys containing lignocelluloses like starch, cellulose acetate etc. U.S. Pat. No. 5,314,934 to Tomka et al. provides a process to produce a polyolefin-starch polymer blend. Ethylene/acrylate/maleic anhydride terpolymer was used as a compatibilizer. These blends were reported to be blown into film with properties comparable to LDPE. U.S. Pat. No. 5,234,977 to Bastioli et al. discloses a material used for the production of biodegradable articles in film, sheet or fiber form, which can be produced by extrusion from a molten mass that includes a synthetic thermoplastic polymer and a destructured starch to which a boron containing compound such as boric acid has been added. U.S. Pat. No. 6,277,899 to Bastioli et al. discloses a polymeric composition comprising filler melt-dispersed in a matrix comprising, a destructurized starch component, a synthetic thermoplastic polymeric component and a fluidizing agent. U.S. Pat. No. 5,412,005 to Bastioli et al. discloses biodegradable polymeric compositions containing a starch based component and a polymeric component, preferably polymers of ethylene-vinyl alcohol or polyvinylalcohol.
U.S. Pat. Nos. 6,235,816 and 6,472,497 describe starch polyester blends.
Other references are:    (1) Ramani Narayan, Steven Bloembergen and Amit Lathia, A Method of Preparing Biodegradable Modified-Starch Moldable Products and Films, U.S. Pat. No. 5,869,647, Feb. 9, 1999, July 1993;    (2) Narayan, R., Biodegradable Multi-Component Polymeric Materials Based on Unmodified Starch-Like Polysaccharides, U.S. Pat. No. 5,500,465, Oct. 31, 1995;    (3) Narayan, R., Krishnan, M., DuBois, P., Polysaccharides Grafted With Aliphatic Polyesters Derived From Cyclic Esters, U.S. Pat. No. 5,540,929, Jul. 30, 1996;    (4) Narayan, R., Krishnan, M., DuBois, P., Polysaccharides Grafted With Aliphatic Polyesters Derived From Cyclic Esters, U.S. Pat. No. 5,578,691, Nov. 26, 1996;    (5) Narayan, R., Krishnan, M., DuBois, P., Polysaccharides Grafted With Aliphatic Polyesters Derived From Cyclic Esters, U.S. Pat. No. 5,616,671, Apr. 1, 1997;    (6) Narayan, R., Balakrishnan, S., Nabar, Y., Shin, B., Dubois, P., Raquez, J., Chemically Modified Plasticized Starch Compositions by Extrusion Processing, U.S. Pat. No. 7,153,354 (U.S. patent application Ser. No. 10/993,309, co-filed with the present application on Nov. 19, 2004 (hereinafter the “copending application”)), which is incorporated herein by reference.